This invention relates to constructions of the forward triangular frame of a bicycle.
This invention relates particularly to the use of steel lug tubes which are tungsten inert gas (TIG) welded and brazed in steel frame sets and which are TIG welded and glued in carbon fiber frame sets.
A brazed, lugged connection is well known and is widely used in traditional, steel bicycle frames.
TIG welding of frame tubes directly to associated frame structures is also well known and widely used in traditional, lugless bicycle frames.
The prior art brazed, lugged frames and the prior art direct TIG welded frames have a number of disadvantages.
The lugs which have been traditionally used to construct lugged steel bicycle frames have incorporated a collar (for encircling a related frame structure, such as, for example, a head tube) in addition to having a short tubular portion which is brazed to a related frame tube.
The traditional, collared lugs were connected to and were brazed to both the frame tube and the related frame structure. Such brazed, collared, lugged connections provided a durable, generally satisfactory, structural connection.
The main disadvantages of the traditional, lugged frame construction have been (1) the overall weight involved in the finished connection, (2) the complex geometry of the collared lug, and (3) the cost involved in producing suitable lugs for different size frames and frequent changes in frame geometries.
Each collared lug at each frame connection location must be complexly curved to achieve the close fit required at the juncture of the frame tube and the related frame structure. A different location of the connection (to accommodate a change in frame size or a change in frame geometry) usually required a different curvature of the surfaces of the collared lug.
The collared lugs have traditionally been made either by complex rolling operations on a sheet of low carbon steel or by individual investment castings. Both lug fabrication techniques required a significant investment for the apparatus needed to produce a specific lug shape and curvature.
When limited production runs of a large number of different frame sizes and frame geometries are required, the traditional, collared, lugged frame construction has drawbacks because of the special apparatus required to fabricate the lugs and the costs associated with the special apparatus required to fabricate the lugs.
Also, the prior art, traditional, lugged frame sets often required a costly, special jig to hold each different size frame set or each different geometry frame set in alignment during brazing.
A relatively large amount of brazing material has also been required to finish a brazed connection in the traditional, collared lugged frames. The brazing material, which is heavy, can make the frame heavier than desired.
The prior art TIG welded frame sets eliminated lugs by TIG welding a frame tube directly to an associated frame part (such as, for example, the head tube).
The disadvantages of the prior art TIG welded frame (as compared to a lugged frame) include less frame strength, less frame stiffness, less frame shock dampening and less frame life.
The strength of a TIG weld area generally is less than the strength of the frame tube alloy material.
The weld area also tends to be brittle, if it is not heat treated.
Heat treating of the TIG welds (in the prior art, direct TIG welded frame set) was so difficult that it usually was not done. Heat treatment of all the welds required exposing the entire frame set to the heat treating temperatures. That could cause the frame set to get out of alignment.
Because of the reduced strength and brittleness in the TIG welded areas, the traditional, TIG welded frame sets usually had a shorter useful life than the traditional, lugged frame sets.
The material is a TIG weld is a hard, unyielding material (as compared to the softer brazing material). The brazing material is relatively elastic and tends to absorb and to dampen road shock. But the direct TIG weld connection does not provide any effective damping.
The TIG weld is essentially a linear connection and cannot distribute forces applied at the connection evenly down the frame.
The brazing material in a lugged connection can distribute forces evenly down the frame.
As a result of all of these factors, the way a bike rides and feels down the road is different with a brazed, lugged frame than it is with a direct TIG welded frame.
TIG welds do not add lateral stiffness to the frame.
Lugs do add lateral stiffness.
The prior art, traditional, TIG welded frame set can also get to be relatively expensive when efforts are made to minimize the weight of a frame set. To reduce the weight of a frame set, the frame tubes are of smaller thickness in the mid part of the tube than at the ends. But relatively large thicknesses at the ends of the tubes are needed to insure adequate thickness for a TIG weld. The frame tubes are usually seamless tubes which are formed by rolling a sheet and seam welding the sheet. However, the tubes are sometimes drawn tubes, instead of being folded over and seam welded tubes. Drawn tubes maximize tube strength and minimize tube weight. But drawn tubes of varied thicknesses at different parts of the tube can become very expensive to make.
It is an important object of the present invention to combine the best features of the prior art lugged frame construction and the prior art TIG welded frame construction, while eliminating or avoiding problems inherent in each of those prior art, traditional frame construction techniques.
A third, traditional, frame construction has incorporated carbon fiber frame tubes and cast aluminum lugs.
The failure point for such carbon fiber bicycle frames usually has not been the carbon fiber.
The failure point has instead usually been the aluminum lugs to which the carbon fiber frame tubes are glued.
The traditional, aluminum lugs have generally been heavy cast pieces which are brittle and which can fail catastrophically if not properly designed.
The cast aluminum lugs are subject to sudden (catastrophic) failure with little or no warning (in contrast to the failure mode of a steel part, which will usually give some prior, warning, high pitched, squealing sound).
It is another important object of the present invention to integrate outer, steel alloy, lug tubes with carbon fiber frame tubes (and with steel inner support tubes) to create structures which are lightweight, yet much stronger and more durable than the prior art, heavy, cast aluminum pieces.
It is a related object of the present invention to combine steel lug tubes with carbon fiber frame tubes in a frame which combines the best qualities of steel alloy lug tubes and carbon fiber frame tubes and to achieve a laterally stiff, yet radially compliant ride for the serious bicycle rider.